The present invention relates to the field of offshore drilling and production structures. More specifically, it relates to an apparatus for a supporting a steel catenary riser (SCR) extending to the seabed from a floating offshore structure (e.g., a platform or a vessel).
Offshore platforms used for the production of petroleum and natural gas from seabed wells frequently include one or more production risers for the extraction from the well of the product (petroleum or natural gas) and/or for the injection into the well of a fluid (gas or water) to aid in the extraction of the product. One type of riser that is well known and that is commonly used is a flexible steel riser known as a steel catenary riser, or “SCR.”
The current practice in riser systems is to provide a pull tube for installing and supporting a steel catenary riser. A support section of the pull tube keeps the stresses in the riser within allowable limits during installation and operation. One prior art example is shown in FIG. 1, which represents an SCR support structure of the type shown in U.S. Pat. No. 5,722,492, the disclosure of which is incorporated herein by reference. A floating offshore structure 12 is held in position in a body of water 36 by mooring lines 14 that are secured to the seabed 16 by means such as anchors or pilings (not shown). An SCR 18 has a lower end attached to a wellhead (not shown) in the seabed 16. A flexible support tube 22 supports the SCR 18 within the structure 12. An upper portion 24 of the riser 18 extends up and through a center well 26 situated within the floating structure 12, for attachment thereto at a location 28.
The SCR 18 is pulled through the support tube 22 by pull-in rigging (not shown). The inside diameter of the support tube 22 must be large enough to allow the pull-in rigging to pass through it. As a result, the support tube 22 diameter is often considerably larger than the diameter of the SCR 18, wherein the relatively large diameter of the support tube 22 relative to that of the SCR 18 gives the support tube 22 a high degree of stiffness relative to the stiffness of the SCR 18. The high degree of stiffness of the support tube 22 (relative to the stiffness of the SCR 18) results in a concentration of bending strain in the SCR 18 at the location where the SCR 18 exits the support tube 22. This concentration of bending strain contributes to excessive stress and strain in the SCR 18.
It would thus be advantageous to provide a catenary riser support system that does not require the use of large diameter risers and support tubes with a large stiffness relative to the stiffness of the riser.